Porous metallic filter assemblies are commonly used in refining technologies to remove solid particles (e.g., catalyst or sorbent particles) from liquid or gaseous hydrocarbon streams. These filter assemblies generally consist of a circular tube sheet comprising about 60-150 filter cartridges attached thereto in a precise pattern.
Over time, the filter cartridges can become plugged due to the accumulation of fine solid particles in the filter pores. Additionally, some refining technologies expose the filters to harsh environments which can cause the filter cartridges to undergo physical and/or chemical transformations. The accumulation of particulate and physical/chemical transformations eventually cause a high pressure drop across the filter assembly, such that the filters can no longer be used effectively. The extent to which the filter is plugged and/or transformed is measured as a differential pressure across the filter bundle such that, when the differential pressure reaches a threshold value (i.e., high alarm), the plugged or dirty filter is removed from the tubesheet. Upon removal, the plugged or dirty filter is either replaced with a new filter or, preferably, cleaned and reused. Replacing plugged or dirty filters presents a serious financial burden due to the high cost of new filters and other costs associated with replacing the filters; therefore, it is preferable to clean and reuse the filters.
However, many conventional cleaning methods cannot remove enough of the solid particles to sufficiently reduce the pressure drop across the filter assembly. Further, these cleaning methods typically require use of harsh chemicals (e.g., acids, bases) and/or conditions that corrode the filter material or otherwise damage the filters. One such method is a vendor based procedure that cleans porous stainless steel filters with 10-25% nitric acid, 10-25% sodium hydroxide at 50 to 75° C. See U.S. Pat. No. 4,493,756. Additionally, some sintered filter materials absorb carbon while in certain hydrocarbon service and become sensitized causing carbide precipitation and chromium depletion in the filter grain boundaries which are susceptible to intergranular attack (i.e., corrosion) by strong acids such as nitric acid. Such corrosion causes the filters to fail (e.g., crack) prematurely.
The cost of such filter assemblies is about $1 million, and the cost of replacing the filter cartridges is about $0.2 to 0.5 million. Accordingly, it is highly desirable to clean plugged or dirty filter cartridges without damaging the filters, and to replace plugged or dirty filters with cleaned filters whenever possible to reduce maintenance costs.
Accordingly, a non-destructive method of cleaning is needed to clean plugged or dirty filter cartridges without corroding or otherwise damaging the filters.